Generally, an ink comprises one or more pigments and a binder enabling the pigment to be put in suspension, transported and fixed on the support which should be covered and on which patterns are to be printed. The choice of the pigment and the binder are essential as the pigment determines the tinting strength of the ink and the binder determines not only the method of drying of the ink, but also the main characteristics (resistance, adherence . . . ) of the ink layer obtained.
Usually, additives are added to the pigment-binder mixture to optimize the characteristics of the ink during and after its application. They are also used to facilitate the implementation of the ink and accelerate drying, improve gloss, and/or increase the strength of the ink film, among other things.
Among the additives for ink, in particular dispersing agents, anti-foaming agents, as well as waxes, polymers, thickeners and plasticizers can be found. Waxes, of animal, vegetal or mineral origin, are used to modify the surface finish of the ink film and are primarily used to increase friction and abrasion resistance and to improve the sliding coefficient of the ink layer. Polymers, generally nitrocellulose- or ethyl cellulose-based, are used to improve the properties of the ink layer, and to increase its gloss and resistance. They are also used to improve the wetting ability of pigments. Thickeners are used to correct the viscosity of the ink.
The composition of an ink varies according to the support on which it is to be applied and the desired result. The ink must exhibit good intrinsic cohesion and good adhesion to the support. All inks cannot be applied to all supports. Often, commonly used inks are not appropriated for application on plastic films or on multilayer products used in particular as decorative surface coatings.
Printing on plastic films is difficult owing to the fact that these films are often manufactured from non-polar polymers, and the application of an ink requires that a coating, also called “primer” is first applied, or the use of a film surface treatment, such as a corona treatment, which has the disadvantage of introducing one or two additional steps into the manufacturing process of such plastic films.
As for the printing of multilayer products, it presents an additional difficulty; indeed, a multilayer product usually comprises a plastic film, also referred to as the substrate layer or support layer, on which another plastic film is applied, also referred to as the upper layer or wear layer. The function of the wear layer, which is usually transparent, is to protect the decorative face owing to its good resistance to mechanical and chemical aggressions that can arise during normal conditions of use of such multilayer products. Generally, the support layer features a decorative side on which the wear layer is applied. As the decorative design is printed before the wear layer is laid, adhesion problems thus appear between this wear layer and the ink, over and above the adhesion problems between the ink and the support layer. The adhesion between the layers of a multilayer product is crucial as it must meet the strictest criteria, in particular in terms of traffic resistance of the final product in the case of a floor covering.
Among plastic films, PVC (polyvinyl chloride) films are very widespread, in particular in the manufacture of all kinds of packing and packaging, but also in the manufacture of wall and floor coverings. PVC products have the major disadvantage of releasing volatile organic compounds and phthalates into the environment. This is why new more ecological products were developed, chlorine-free products, particularly olefin polymer and copolymer-based products having essentially identical thermal, mechanical and chemical properties, products that are easier to manufacture, less expensive and more ecological to manufacture than PVCs, and that release less volatile organic compounds. Nevertheless, the use of olefin polymers does not solve the adhesion problem between ink and plastic, nor does it provide a solution to the adhesion problem between the support layer and wear layer encountered in the manufacture of multilayer products.
Usually, the surface of the polyolefin film (or the polyolefin support layer in the case of a multilayer product) to be decorated, must be treated before the decorative design is printed. This may involve the application of a “primer” or a physical treatment, particularly a corona treatment as described in document FR2836088.
In the case of a multilayer product, as described in document FR2836088, an adhesion promoter must be applied to the decorative layer in order to improve adhesion between the ink layer and the wear layer. The adhesion promoters cited in document FR2836088 are polyolefin, chlorinated polyolefin, maleic anhydride or hydroxy-functional polyolefin or epoxy-based solutions, acrylic, cross-linked polyester-based solutions or silane-based solutions.
For multilayer products, an alternative solution consists in using an adhesive ink for which the adhesion promoter is an integral part of the ink.
Document EP0604729 describes an adhesive ink for printing on a polyolefin polymer-based support in order to obtain woven and non-woven products in the form of fibers or films. This ink comprises a pigment, a binder, and an adhesion promoter. The binder is either polyvinyl alcohol or ethylene-vinyl acetate (EVA), which are compounds used for their film forming ability, while the adhesion promoter is a compound chosen among hydrocarbon resins, colophane esters or polyterpenes.
The disadvantage of such adhesive inks it that their formulation is complex and not sufficiently adherent to the polyolefin support on which it is applied, as their formulation represents a compromise between the homogeneity of the ink and adhesion to the support. This compromise places their adhesion property as a disadvantage.